As a polymer material having elasticity, those obtained by mixing rubbers such as natural and synthetic rubbers with crosslinking agents and reinforcers and crosslinking the mixture at high temperature under high pressure have commonly been used, heretofore. However, these rubbers require the step of crosslinking and molding at high temperature under high pressure for a long time and are therefore inferior in processability. Since the crosslinked rubbers exhibit no thermoplasticity, recycle molding can not be conducted, unlike thermoplastic resins. Therefore, there have recently been developed various thermoplastic elastomers which can be easily processed into formed articles by employing common melt-molding techniques such as hot-press molding, injection molding, and extrusion molding in the same manner as in case of conventional thermoplastic resins. As these thermoplastic elastomers, various polymers such as olefinic, urethane, ester, styrene and vinyl chloride polymers have been developed and are put on the market at present.
Among these polymers, styrenic thermoplastic elastomers are highly flexible and are excellent in rubber elasticity at normal temperature. As these styrenic thermoplastic elastomers, for example, there have been developed styrene-butadiene-styrene block copolymer (SBS) and styrene-isoprene-styrene block copolymer (SIS); styrene-ethylenebutylene-styrene block copolymer (SEBS) and styrene-ethylenepropylene-styrene block copolymer (SEPS) obtained by hydrogenating the above polymers. However, these block copolymers were insufficient in permanent compression set.
Also Kohyo Publication (National Publication of Translated Version) of WO93/14135 discloses an isobutylene block copolymer comprising a polymer block composed mainly of isobutylene and a polymer block composed mainly of an aromatic vinyl compound as a thermoplastic elastomer which is highly flexible and is excellent in rubber elasticity at normal temperature, and is also excellent in vibration damping properties, gas barrier properties and hermetical sealing properties as features of an isobutylene polymer. However, this isobutylene block copolymer is also insufficient in permanent compression set, like the above-described styrenic thermoplastic elastomers.
As a technique for improving the permanent compression set of this isobutylene block copolymer, Kohyo Publication (National Publication of Translated Version) of WO98/14518 discloses a thermoplastic polymer composition comprising an isobutylene block copolymer containing a polymer block composed mainly of isobutylene, and crosslinked article of a rubber, while Japanese Unexamined Patent Publication (Kokai) No. 11-293083 discloses a composition comprising an isobutylene block copolymer, a crystalline polyolefin and a plasticizer (softener). Although these composition have improved permanent compression set while maintaining features of the isobutylene polymer, thermoplastic elastomer compositions having more excellent permanent compression set are required.